Author Topic: Fractal and Chaos Properties of Explosion Earthquakes Followed by Harmonic Tremo  (Read 1745 times)

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Author : Sukir Maryanto, Didik R. Santosa, Iyan Mulyana, Muhamad Hendrasto
International Journal of Scientific & Engineering Research Volume 2, Issue 6, June-2011
ISSN 2229-5518
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Abstract— Fractal and chaos analyses of successive event of explosion earthquake and harmonic tremor recorded at Semeru volcano were applied to investigate the dynamical system regarding to their eruptive mechanism. The eruptive activity at Semeru volcano East Java, Indonesia is intermittent emission of ash and bombs with Strombolian style which occurred at interval of 15 to 45 minutes. The explosive eruptions accompanied by explosion earthquakes and followed by harmonic tremor which generated by continuous emission of volcanic ash. Spectral, fractal and Lyapunov exponent of successive event of explosion and harmonic tremor were analyzed. Peak frequencies of explosion earthquakes range 1.2 to 1.9 Hz and those of the harmonic tremor have peak frequency range 1.5 – 2.2 Hz. Temporal change of fractal dimension shows a higher fractal dimension of explosion earthquakes and gradually decrease during the occurrence of harmonic tremors. The phase space is reconstructed and evaluated based on the Lyapunov exponents. Harmonic tremors have smaller Lyapunov exponent than explosion earthquakes. It can be considerably as correlated complexity of the mechanism from the variance of spectral and fractal dimension and can be concluded that the successive event of harmonic tremor and explosions are chaotic. 
Index Terms— Semeru volcano, explosion earthquakes, harmonic tremor, spectral, fractal dimension, lyapunov exponent.

1   INTRODUCTION
SEMERU volcano is an andesitic stratovolcano located in East Java (Fig. 1). The peak’s summit rises 3676 m above sea level and it is the highest, active volcano on the island of Java. Frequent explosions at the summit crater of Jonggring Seloko have been occurring continuously since 1941.[1],[2] During this period, small to moderate strombolian and vulcanian type explosions have occurred, producing explosion plumes rising 400–1000 m above the summit. During active periods, lava flow, lava dome extrusion and pyroclastic flows have also been observed. In 2005 the frequency of the explosions averaged 3453 times per month and 115 per day, respectively.[3],[4]
In previous study, Maryanto et al., 2005, classified harmonic tremor at Sakurajima volcano into 2 groups: Harmonic Tremors occurred after B-type earthquake swarms (HTB) and Harmonic Tremor which occurred immediately after Eruption (HTE). There is no study focus on harmonic tremor after an eruption at Semeru volcano previously. We have analyzed Harmonic Tremors after an Eruption (HTE) that occurred at Semeru volcano on March 2005. HTEs that occurred at Sakurajima volcano were characterized to understand more detail volcanic activity as supporting information for prediction of volcanic eruption.[5]
 Most of the previous study mentioned above are mainly based on the characteristics of spectra which mostly triggered by linear process, however little intent has been given on nonlinear process. In contrary, theoretically volcanic tremors can be generated by some kind of nonlinear processes.[6] Methods based on the discipline of nonlinear dynamics have been rarely applied to the volcanic tremor and explosion earthquake recorded at some volcanoes. The first investigation of fractal properties of volcanic signals have been conducted for volcanic tremor for the tremor and gas piston events recorded at Kilauea volcano, Hawaii.[7] The results revealed a fractal dimension of the tremor attractor in the range of 3.1-4.1 with the average value of 3.75. This was interpreted that the source of tremor is not controlled by a stochastic process (where fractal dimension should be infinite), therefore it can be described by only a few degrees of freedom. Some studies of tremor and low-frequency events reported similar estimates of the fractal dimensions confirming the low-dimensional nature of the phe-nomena.[6],[7],[8],[9],[10] Furthermore, the time interval between two successive explosions can be considered as a dynamical variable.
In the present paper, another approach of fractal and chaos analyses of the explosion earthquakes followed by harmonic tremor has been applied. We applied the method to successive event of explosion earthquakes and harmonic tremors recorded at Semeru volcano, East Java, Indonesia. First, we briefly describe the data ob-tained from Center for Volcanology and Geological Hazard Mitigation, Indonesia. We then present a method to estimate the fractal dimension and Lyapunov exponent of the explosion earthquakes and harmonic tremor. We follow with an application of the method to our data and terminate with a discussion of the implications of these results for driving mechanism of the events.

2   OBSERVATION
Fig. 1. Map of Semeru volcano and locations of seismic stations at this volcano operated by Center of Volcanology and Geological Hazard Mitigation. Triangle, circles and squares represent summit crater, temporary and permanent stations, respectively. In this study, we used permanent stations (BES and LEK).
Semeru volcano has been monitored continuously by CVGHM using 5 seismometers at stations PCK, KPL, TRS, LEK and BES, distributed 0.76-8.9 km apart from the active crater. Seismic sensor installed at the summit and northwest-south slope (Fig. 1). The 5 stations are equipped with short-period (1 Hz) vertical seismometer. The temporary stations (PCK, KPL, and TRS) installed during September – December 2005. The signals from seismometers transmitted to the Sawur Volcano Observatory by FM radio telemetry. The recorded seismic signals transmitted to the G. Sawur Volcano Observatory by FM radio telemetry. [11]

The seismic signals are recorded by analog drum recorders; Kinemetrics PS-2 and digitally sampled 100 Hz by data loggers (Datamark LS-7000) with GPS time calibration. Characteristic signals such as volcanic earthquakes, tectonic earthquakes, and eruption earthquakes also harmonic tremor are recognized on the seismograms. The selected successive events of explosion earthquakes and harmonic tremor are represented in Table I. The harmonic tremor occurred about 2-3 minutes after explosion.

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